The present invention relates to a piezoelectric actuator.
A piezoelectric actuator for adjusting an ultra-fast-acting valve is described in German Published Patent Application No. 37 136 97, for example.
FIG. 1 shows a sectional view of a piezoelectric actuator 1 known in the related art. This piezoelectric actuator has disks 2 made of a piezoelectric material, arranged one above the other in the form of a stack 25. Inside electrodes 5 and 6 are arranged between successive disks 2 made of piezoelectric material. Inside electrodes 5, 6 and outside electrodes 3 and 4 arranged on the outside of stack 25 of disks 2 made of piezoelectric material contact one another. Outside electrodes 3 and 4 are each connected electrically to a terminal 11a and 11b of a control voltage source 10. Inside electrodes 5 and 6 are thus connected alternately to terminals 11a and 11b of control voltage source 10 across outside electrodes 3 and 4.
FIGS. 2A and 2B show enlarged views of a circular detail of an edge area of piezoelectric actuator 1 shown in FIG. 1. In the edge area shown in the enlargement, a distinction should be made between area 7 and area 8. In area 7, inside electrode 5 is not contacted through to outside electrode 4 in area 7, and disks 2 made of piezoelectric material are sintered through in area 7. In area 8, inside electrode 6 is connected to outside electrode. 4. The adhesive strength in area 8 between a disk 2 made of piezoelectric material and an inside electrode 6 is smaller by a factor of 3 to 5 than the adhesive strength in area 7 between sintered disks 2 made of piezoelectric material.
FIG. 2A shows the condition of piezoelectric actuator 1 without an electric field E being applied. FIG. 2B shows expansion in the central area of actuator 1 because of the piezoelectric effect after a control voltage has been applied. However, there is no piezoelectric expansion in edge area 7, where there are no oppositely polarized inside electrodes arranged opposite one another, but instead disks 2 made of piezoelectric material are sintered through. Due to the expansion in the central area of piezoelectric actuator 1, tensile stresses occur in the edge area, frequently resulting in delamination 9 in the contact zone between inside electrodes 6 and disks 2 made of piezoelectric material in area 8.
As shown in FIG. 2B, this delamination 9 continues into the area of outside electrode 4. Such a crack 9xe2x80x2 propagating into outside electrode 4 has a considerable negative effect on the contacting of inside electrodes 6. If crack 9xe2x80x2 propagates completely through outside electrode 4, the contact in outside electrode 4 is interrupted and a portion of inside electrodes 6 no longer receives voltage. This greatly reduces the total expansion, i.e., the travel of actuator 1.
Various proposals have been made in the related art to guarantee continued functionality of actuator 1 when cracks 9xe2x80x2 occur.
The piezoelectric actuator has the advantage over the related art that the outside electrodes applied in the form of a coating of electrically conductive polymer material on the outside of the stack are elastic. In the case of the actuator according to the present invention, tensile stresses occurring in the edge area of an outside electrode due to an applied voltage are absorbed by the elastic outside electrode without any tears or cracks occurring in the outside electrode. If a crack develops between the inside electrode and a disk made of piezoelectric material, it is subsequently stopped at the elastic outside electrode. A piezoelectric actuator provided with elastic outside electrodes solves the problem of cracks occurring in the outside electrodes of piezoelectric actuators in a simple and reliable manner.
In addition, it is extremely advantageous that this eliminates the etching and seeding of the outside surface of the actuator, which has previously been necessary in the manufacture of a conventional actuator 1 and is necessary in order to apply a nickel outside electrode, for example. With the etching required in the related art, there is always the risk of undercutting and subsequent detachment of the outside electrode in the operating state of the actuator.